Several anticancer drugs were tested for induction of cytolysis mediated by poly-morphonuclear leukocytes (PMNs). Among them, actinomycin D and vincristine sulfate induced tumor lysis mediated by PMNs (drug-dependent cellular cytotoxicity). The cell-free supernatant of PMNs incubated with actinomycin D in vitro could lyse various murine tumor cells, although normal spleen cells and sheep red blood cells were not lysed. The cytolytic activity of the supernatant was resistant to treatment with superoxide dismutase, catalase, trypsin inhibitor or arginine. However, the cytolytic activity was labile to heat treatment at 56°(30min) or trypsin treatment, suggesting a protein nature. These results suggest that PMNs can lyse tumor cells in the presence of certain anticancer drugs and that a factor(s) from PMNs may participate in the killing of tumor cells.